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9.12 CHAPTER NINE

quality and filter run lengths. However, based on concerns about microbiological conta-
minants, it is prudent to design filter rates conservatively to allow flexibility for future in-
creased treatment requirements and fluctuating demands for water quantity and quality.
One design approach is to allow a moderate to low rate, even when one filter is off-
line and the treatment system is under maximum water demand conditions. This allows
some flexibility to meet future increased demands without capital expenditures and pro-
vides a buffer under poor source water quality conditions. This approach also increases
initial capital expenditures because of larger filter area requirements.
Some cost savings may be gained by allowing on-line filters to operate at high rates
when one unit is off-line, but research has shown that even though the schmutzdecke has
been established, an increase in filter rate can have an adverse impact on treated water
quality. The higher rate also results in reduced length of filter runs.
A compromise approach is to provide moderate flow rates when all units are in ser-
vice. Higher rates result with one unit off-line, but only for short periods. This provides
flexibility when dealing with water quality and demand issues and reduces capital
expenditures.

Automation. Slow sand filtration plants can require much simpler automation than other
filtration processes. But to some degree, the level of automation is driven by regulations
and the client. For automation, the design must consider

• State and federal regulations
• Initial cost and operating costs
• Level of complexity desired by client and operator
• Work shifts to be used
• Source water quality fluctuations
• Record-keeping needs and requirements
• Critical plant parameters and equipment
The recommended minimum location points for recording and monitoring include
• Source water turbidity
• Source water flow
• Filter level
• Filtered water flow and head loss (individual)
• Filtered water turbidity (individual or combined)
• Chlorine residual (before and after storage)
• Finished water flow
• Finished water storage level
Other monitoring points may be specific to each site's requirements and should be dis-
cussed with the client.

Pilot Testing
The most advantageous way of determining the applicability of a treatment process to a
specific source water and the performance that may be expected is through pilot studies.
In fact, reliable prediction of treatment results is difficult without pilot testing. The re-